(1) Field of the Invention
The present invention relates to a dispersion compensating device and an optical transmission system, and more particularly to a technique suitable for a dispersion compensating device and an optical transmission system using VIPA (Virtually Imaged Phased Array).
(2) Description of Related Art
In an optical transmission system, dispersion compensation must be carried out when the wavelength dispersion of a transmission line is large. As a dispersion compensator, a fiber type (what is known as DCM) is general. However, in recent years, a dispersion compensator that is not a fiber type, such as VIPA, Ethalon, Fiber Bragg Grating (FBG), or waveguide resonance type, has been realized. Here, as a device using the VIPA, a technique proposed by the later-mentioned Japanese Patent Application Laid-Open No. 2003-294999 is known, for example.
Among these, the VIPA enables dispersion compensation with a simple and compact construction, and also the dispersion compensation amount can be varied, so that the VIPA is a highly prospective dispersion compensating device. However, since the VIPA has a structure that uses resonance, the VIPA has a property such that the pass band in which the dispersion compensation can be carried out is periodic, and the pass band width at each wavelength is limited (pass band is narrow). For example, FIG. 19 is a model view showing one example of pass band characteristics of the VIPA. As illustrated in the top of FIG. 19, the peaks (center wavelengths) of the pass band characteristics (which may hereafter be simply referred to as “passage characteristics”) appear periodically at an extremely narrow interval such as 50, 100, or 200 GHz. The bottom of FIG. 19 shows the group delay characteristics to the wavelength, and shows how the group delay is shifted from 0 according as the wavelength is shifted from the aforementioned peaks. Additionally, as a technique related to VIPA, technique proposed in Japanese Patent Application Laid-Open No. 2003-294999.
For this reason, in a non-WDM (Wavelength Division Multiplexing) system, a dispersion compensator (DCM) whose pass band is a wide band is generally used instead of using a periodic wavelength dispersion compensator [hereafter referred to as periodic (or periodic type) dispersion compensator] in which the pass band where the dispersion compensation can be carried out is a narrow band and the peaks of transmittance repeatedly appears at a predetermined interval, such as VIPA or Ethalon filter.
On the other hand, when a periodic dispersion compensator is to be used in a WDM transmission system, since the pass band in which the dispersion compensation can be carried out is a narrow band and periodic as described above, the transmission wavelength of a light source (optical transmitter) must be stabilized at a high precision to the grid wavelength of ITU (International Telecommunication Union) standard (hereafter referred to as ITU grid wavelength) λItu, and the transmittance wavelength (pass band characteristics) of the periodic dispersion compensator must be stabilized to the ITU grid wavelength λItu. For this reason, as shown in FIG. 20, an optical transmitter 100 is provided with a wavelength detecting circuit 103, an LD temperature controlling circuit 104, and the like for wavelength stabilization (wavelength lock) in addition to an LD module 101 incorporating a light source (LD) 1010 such as a semiconductor laser and a wavelength variation detecting circuit 1011, and an LD current controlling circuit 102, whereby the wavelength variation (error) is detected by receiving the wavelength variation information obtained by the wavelength variation detecting circuit 1011 with the wavelength detecting circuit 103, and the transmission wavelength of the optical transmitter 100 is stably equalized to the corresponding ITU grid wavelength by controlling the temperature of the light source 101 (for example, controlling the Peltier element provided in the light source 1010) with the LD temperature controlling circuit 104 so that the detection error will be minimized. On the other hand, the passage characteristics of periodic dispersion compensator 200 is stabilized to the ITU grid wavelength by temperature stabilization or the like.
In this manner, by sufficiently equalizing and stabilizing both the transmission wavelength of the optical transmitter 100 and the pass band characteristics of the periodic dispersion compensator 200 to the ITU grid wavelength, stable dispersion compensation characteristics can be obtained. Here, in FIG. 20, reference numeral 105 represents an external modulator (for example, LN modulator or the like) that modulates the light from the light source 101 with a transmission signal (data). This external modulator 105 is not needed in the case of a direct modulation system. Further, the arrow shown with a thick solid line represents an electric signal line, and the arrow shown with a thin solid line represents an optical signal line.
Here, as a conventional technique related to wavelength stabilization, techniques proposed in Japanese Patent Application Laid-Open No. 2000-323784, Republication of International Laid-open Patent Publication WO97/34379, and Japanese Patent Application Laid-Open No. 2003-218461, and Japanese Patent Application Laid-Open No. 2003-294999 are known, for example.
Here, the technique of Japanese Patent Application Laid-Open No. 2000-323784 provides a multiple wavelength stabilizing apparatus in which, when a tunable laser is used as a spare laser, all of the plural wavelengths that can be output from the tunable laser can be stabilized, and also the drawing range can be widened. For this reason, the multiple wavelength stabilizing apparatus of Japanese Patent Application Laid-Open No. 2000-323784 is constructed to include an interferometer that makes the incident light interfere at a period corresponding to double the wavelength interval of the channel in the WDM system and outputs the interference light through two ports by shifting at half the period, first and second detecting means for respectively detecting the output light intensities from the aforesaid ports, and controlling means for determining whether the channel fixed to a predetermined wavelength is even or odd and performing control so that the output wavelength of the laser light source will be the predetermined wavelength on the basis of the determination result and the output of the aforesaid detecting means.
Then, in this multiple wavelength stabilizing apparatus, the output wavelength of the laser light source can be fixed to the predetermined wavelength by determining whether the channel of the predetermined wavelength is an even channel or an odd channel and giving a controlling signal to the laser light source so that the detected ratio value (PDo1/PDo2) of the output (PDo1) of the first detecting means divided by the output (PDo2) of the second detecting means will be a target value. Further, since the same value of PDo1/PDo2 appears at a period of double the channel wavelength interval respectively between the even channels and between the odd channels, the drawing range of each channel can be made to be double the channel wavelength interval with its center being at the predetermined wavelength.
The technique of Republication of International Laid-Open Patent Publication WO97/34379 relates to an optical transmission apparatus in which an optical fiber grating (FBG) is used for dispersion compensation, where a dispersion compensation FBG having a narrow band is disposed in a transmitter, and a dispersion compensation FBG in which the center wavelength is set in advance to be equal to the center wavelength of the transmitter side FBG at a center temperature of use is disposed in a receiver. Then, on the transmitting side, the wavelength of the transmission light source is stabilized to the center wavelength of the transmitting side FBG with a wavelength stabilizing circuit, and simultaneously the dispersion compensation is carried out, whereas on the receiving side, the dispersion compensation is carried out by the receiving side FBG, so as to suppress the deterioration caused by the self phase modulation effect (SPM). Further, by setting the wavelength band width of the aforesaid transmitting side FBG to be narrower than the wavelength band width of the receiving side FBG, the transmission wavelength can be made to fall within the reflection band of the receiving side FBG even if a temperature change occurs independently on the transmitting side and on the receiving side. Also, the wavelength band width required in the receiving side FBG can be reduced.
Furthermore, the technique of Japanese Patent Application Laid-Open No. 2003-218461 relates to a method and a system that enables wavelength stabilization with a simple construction by using QCSE photodetection that can serve both as a filter and a detector, where the photocurrent of the emitted light from one light source is detected respectively by the first and second QCSE photodetectors that operate by receiving supply of different bias voltages, and the output wavelength of the light source can be stabilized to a predetermined wavelength by controlling the light source so that the detected photocurrents may be equal to each other.
However, as described above, since a periodic dispersion compensator has a restricted pass band (narrow band) relative to the wavelength, the wavelengths of the light source and the dispersion compensator must be equalized at a high precision and, as a technique therefor, it is necessary to devise to stabilize the temperature of the dispersion compensator and also to devise to stabilize the light source by incorporating a wavelength locking function therein. As a result of this, the light source and the dispersion compensator both will have a complex construction, thereby raising a problem of high costs.
Also, in a long distance transmission system of non-WDM, since a light source that does not need to be stabilized to the ITU grid wavelength is used, there is a problem in that a dispersion compensator having periodic pass band characteristics cannot be usually applied.
Further, also in a WDM transmission system, it is inefficient as described above to perform stabilization both on the light source side and on the dispersion compensator side and, when numerous dispersion compensators are used in the system, wavelength stability of higher precision is required. Also, in a long distance transmission system of WDM, when periodic wavelength dispersion compensators are to be used in plural optical relay nodes constituting the system, the wavelength stabilization of the dispersion compensators of all the nodes and the wavelength stabilization of the transmitting light source must be carried out individually for all. This is not preferable because the system as a whole will be highly costly.
Also, since the techniques of the aforementioned Japanese Patent Application Laid-Open No. 2000-323784, and Japanese Patent Application Laid-Open No. 2003-218461 are directed to a technique of wavelength stabilization singly on the transmission side, so that the relationship between the transmission wavelength and the pass band characteristics of the dispersion compensator is not considered at all. On the other hand, according to the technique of the aforementioned Republication of International Laid-open Patent Publication WO97/34379, the light source and the dispersion compensator both will not have a complex construction because the transmission wavelength is stabilized to the center wavelength of the transmitting side narrow band FBG disposed in the transmitter and having a dispersion compensating function, as described above. However, various problems are raised because the output wavelength of the light source is controlled.
Namely, in order to control the output wavelength of the light source, it is general to control the temperature by using a Peltier element or the like. However, the electric power consumption will increase, and a large load is imposed on the light source depending on the variation range of the output wavelength, which may become a factor inviting decrease of the life time of the light source or generation of abnormality. Also, when the center light-emission wavelength is changed, an unexpected output power variation may occur, thereby raising a fear of giving adverse effects on the overall system. Further, in the case of a WDM transmission system, since it is general to stabilize the output wavelength of the light source to the ITU grid wavelength as already described, a technique of changing the output center light-emission wavelength of the light source such as disclosed in the aforementioned Republication of International Laid-Open Patent Publication WO97/34379 cannot be applied.
In addition, according to the technique of the aforementioned Republication of International Laid-Open Patent Publication WO97/34379, an optical coupler must be inserted in the main signal transmission system (output fiber) to monitor the output light (main signal light) of the dispersion compensator (FBG) for wavelength stabilization. The insertion loss generated by this will equivalently be the loss of the dispersion compensator, thereby raising a problem of increase in the loss.